Due to increasing concern about the effects of certain pollutants that result from coal burning, alternate methods for burning coal are being investigated. One of the methods that is employed to reduce the production of oxides of sulfur uses a fluidized bed, in which the force of the flowing combustion air is used to keep coal particles in a quasi-liquid state. Limestone particles are included in the bed, and the heat of combustion causes the limestone to give off some carbon dioxide, forming quicklime. Quicklime reacts with sulfur trioxide or oxygen and sulfur dioxide to produce a calcium sulfate coating on the limestone. The sulfur content of the stack emissions is thereby reduced.
The operation of the fluidized bed has certain drawbacks, one in particular being the amount of dust loading of the flue gas, which is one or two orders of magnitude greater than that which characterizes conventional pulverized coal-firing. This dust loading necessitates incorporating large centrifugal separators, and baghouses to reduce plant particulate emission to or below EPA limits. The high dust loading greatly increases the wear experienced by equipment such as superheater and reheater tubes, air heaters, and so on. In addition, part of the dust is unburned fuel having heating value, so efficiency is lost unless the dust is reclaimed. But the recycling of the dust necessitates that large ducts be provided to transport the dust around the system, and this increases capital expenditures.